‘No. Only a choirboy,’ said William, grinning. ‘In any case, if both of us went missing, it’s possible the guide would notice and come looking for us.’

‘He didn’t count us before he began the tour.’

‘Well spotted,’ said William, touching his forehead in mock salute. ‘Nevertheless, when the tour ends, I want you to come back here and brief our boys, who will be waiting in squad cars ready to move at a moment’s notice should Faulkner appear. Anything else the guide said that was particularly revealing?’

‘The gates of the grounds will be closed at one o’clock.’

‘Which suggests there’ll only be one more tour today. So let’s get moving, because we can’t afford to miss it.’

They walked swiftly back down the slope towards the castle. Once they were inside, William purchased two more tickets from an elderly lady seated at the reception desk. They then joined a dozen or so people who were assembled in the hall waiting for the tour to begin.

Without a word, Ross made his way to the front of the group, while William remained near the back. Once the guide had given his introductory remarks, the tour began. William couldn’t resist stopping to admire several of the collection’s gems as they moved from room to room. He looked forward to telling Beth about a Farquharson, a Raeburn and a Peploe when he returned home that evening. He didn’t revert to being a detective until they were back in the dining room.

Ross remained at the front of the group while the guide told them how the first Lord McLaren had acquired Fishers of Men over two hundred years ago.

William pretended to be looking at a portrait by one of Caravaggio’s less familiar contemporaries as he drifted casually towards the spiral staircase that led up to the minstrels’ gallery. The guide concluded his remarks on the centrepiece of the collection, and began to walk towards the next room. A few worshippers couldn’t resist one last look at the masterpiece before they rejoined the rest of the group.

Once William was certain he was alone, he stepped nimbly over the rope and made his way up the spiral staircase to the gallery. One or two creaks caused him to look back and make sure no one had spotted him. On reaching the gallery, he moved swiftly around the dogleg, then tucked himself up against the far side of the organ.

Although he had a clear view out of the large bay window, he couldn’t see the dining room table below, or the Caravaggio. He settled back to do what he’d done so many times in the past: sit, wait, be patient and whatever you do, don’t lose your concentration.

When the guide came to the end of the tour, Ross was among the first to break away from the group and quickly leave the castle. He noted that, although the guide mentioned the shop and the café, this time he didn’t suggest that they should feel free to roam around the grounds. However, he did remind them that the gates would be closing at one o’clock.